[SOLVED] I ended up just using the morphClass and an alias to match the morphMany, instead of the Image->getReferenceTypeAttribute() method. It turns out the 502 error was the morphMany working, which triggered another issue happening down the line (I had removed to simplify the code). Here are the changes that were made to the above code examples.

UserCharacter Model (7/8)

class UserCharacter extends Model
{
    protected $morphClass = 'UserCharater' // was cosplayheroes\UserCharacter.  The 502 error was actually the morphMany working, but other code issues down the line.

    ....
}

Image Model (8/8)

class Image extends Model
{
    ...
    
    //removed protected $types = [ ... ];

    ....

    //removed public function getReferenceTypeAttribute($type) { ... }

}

config\app.php (9/8)

    ... 

    'aliases' => [
        ...
        'UserCharacter' => 'app\UserCharacter', /* added this alias so morphClass and the MySQL reference_type would match */
    ],

And, of course, I can't forget that animated GIF reward I promised myself...